Aerosol neutralizers are utilized in a variety of aerosol application and test devices, including characterization of aerosols that are sparsely populated (e.g. the monitoring of clean room environments) as well as aerosols that are particle laden (e.g. combustion engine exhaust, coating sprays). The primary task of the aerosol neutralizer is to condition the aerosol to obtain a reproducible, steady-state population of particles having a distribution of charged (positive and negative) particles and neutral particles that is known to within an acceptable uncertainty, and to produce such a characteristic in the aerosol regardless of the charge condition of the aerosol entering the aerosol neutralizer. By conditioning the aerosol to a known steady-state population, the total concentration of particles can be inferred by measuring only a portion of the particle distribution (e.g., particles of a certain size, mobility, and/or charge).
For example, some detectors detect only positively charged particles within a given size range. Because the steady state population of particles is reasonably known relative to the positively charged particles within the size range, the total concentration of the particles can be inferred. An unconditioned aerosol stream may not possess the steady state characteristics, thus rendering the inference meaningless.
One way to condition an aerosol is to bombard it with x-rays. The x-rays interact with the gas in the aerosol, producing a bi-polar population of ions. These ions then interact with the particles of the aerosol, thereby transferring charge to the particles. Particles having a high charge upon entering the aerosol neutralizer will attract oppositely charge ions generated in the gas, thus tending to neutralize the particle. By this same mechanism, most of the smaller particles will not sustain multiple charges of a given polarity. Larger particles can sustain multiple charges by virtue of their size. See Wiedensohler, “An Approximation of the Bipolar Charge Distribution for Particles in the Submicron Size Range,” J. Aerosol Sci., vol. 19, no. 3, pp. 387-389, 1988. Accordingly, the steady state distribution of the aerosol population comprising a mixture of neutral particles, single-charged particles and multiple-charged particles is rapidly attained under x-ray bombardment.
Conventional neutralizing devices utilizing radioactive substances such as americium (241Am), krypton (85Kr), polonium (210Po) and the like are known to produce a bipolar population of charged particles in an aerosol. Such devices carry with them concerns stemming from the hazardous radiation attendant the radioactive substance and from the gradual decrease of effectiveness characterized by the half-life. Americum has a half-life of 432 years and krypton a half-life of 11 years, thus posing safety concerns both in terms of personnel utilizing and storing the device, and in terms of disposal of the unit when its operational life is at an end. Polonium has a substantially shorter half life (138 days), which may mitigate against long term disposal concerns, but presents additional handling concerns as the radioactive substance typically requires replacement during the operational life of the neutralizer.
Recently, U.S. Patent Application Publication No. 2006/0108537 disclosed an aerosol particle charging device that utilizes “soft” x-rays, that is, x-rays having a wavelength in the range of approximately 0.13- to 2-nm and having photon energies in the range of approximately 600- to 10,000-electron-volts (eV). Soft x-ray devices eliminate concerns regarding handling of radioactive materials because the soft x-ray emitter does not utilize or produce radioactive materials nor does it emit any radiation when no power is supplied to it.
However, both the conventional and the soft x-ray devices are known to generate particles by a process of radiolytic precipitation. Radiolytic precipitation is the result of a cascade of events beginning with ionization of individual molecules which form reactive species from gas constituents and impurities in an aerosol, subsequently interacting with each other to condense into particles. Hence, aerosol neutralizers utilizing x-ray devices typically add particles to an aerosol stream via radiolytically produced particle. The generated particles are generally undesirable, as they typically add particles of unknown size and composition to an aerosol, thereby distorting the aerosol that is being characterized.
A neutralizer that provides the radioactivity-free operation of the soft x-ray emitter without significant radiolytic generation of particles would be welcome.